Infrared imaging of faculae at the deepest photospheric layers

Abstract

Abstract : We have used the NOAO 58 x 62 InSb array and the National Solar Observatory McMath telescope to image the deepest photospheric layers of three active regions at the 1.63 micron opacity minimum. At 1.63 microns, faculae are darker than the photosphere, with a measured contrast of at least 2% at positions on the disk between approx. 0.75<mu<1.0. Near the limb, at mu <0.5, they are brighter than the photosphere, as in the visible. At intermediate 0.5< or = mu < or = 0.75, they are difficult to detect at 1.63 microns. The observation that faculae and their immediate surroundings exhibit a clear deficit of brightness temperature near disk center at 1.63 microns seems to rule out the 'hillock' model put forward to explain their center-to-limb contrast variation. The transition from bright faculae at mu = approx. 1 in visible continuum, to dark faculae at 1.63 microns seems to be explained by existing 'hot wall' models of these structures. We also discuss the imperfect spatial correspondence found between dark faculae and bright Ca K plage in terms of possible differences in size between photospheric flux tubes that give rise to Ca K plages.